1. Field of the Invention
One embodiment of the present invention relates to a light-emitting element, or a light-emitting device, a display device, an electronic device, and a lighting device each including the light-emitting element.
Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, one embodiment of the present invention relates to a process, a machine, manufacture, or a composition of matter. Specifically, examples of the technical field of one embodiment of the present invention disclosed in this specification include a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a lighting device, a power storage device, a memory device, a method for driving any of them, and a method for manufacturing any of them.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance (an EL layer) is interposed between a pair of electrodes. By applying a voltage between the pair of electrodes of this element, light emission from the light-emitting substance can be obtained.
Since the above light-emitting element is a self-luminous type, a light-emitting device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Further, the light-emitting device using the light-emitting element also has advantages in that it can be formed to be thin and lightweight, and has high response speed.
In the case where the above light-emitting element is used for a display panel, there are the following two methods: a method of providing subpixels in a pixel with EL layers having functions of emitting light of different colors (hereinafter referred to as a separate coloring method) and a method of providing subpixels in a pixel with, for example, a common EL layer having a function of emitting white light and color filters each having a function of transmitting light of a different color (hereinafter referred to as a white EL+color filter method, and it is noted that the emission color of the common EL layer is not limited to white).
One of the advantages of the white EL+color filter method is that the EL layer can be shared by all of the subpixels. Therefore, compared with the separate coloring method, loss of a material of the EL layer and cost of patterning for forming the EL layer can be reduced; thus, display devices can be manufactured at low cost with high productivity. In the separate coloring method, it is necessary to provide a space between the subpixels to prevent mixture of the materials of the EL layers in the subpixels. In contrast, the white EL+color filter method has another advantage in that the space is not necessary and therefore a high-resolution display device having higher pixel density can be achieved.
The light-emitting element can emit light of a variety of colors depending on the kind of light-emitting substance included in the EL layer. A light-emitting element which can emit white light or light of color close to white with high efficiency has been particularly required to be applied to a lighting and a display device utilizing the white EL+color filter method. In addition, a light-emitting element having low power consumption is required.
In order to improve the extraction efficiency of light from a light-emitting element, a method has been proposed, in which a micro optical resonator (microcavity) structure utilizing a resonant effect of light between a pair of electrodes is used to increase the intensity of light having a specific wavelength (e.g., see Patent Document 1).
Furthermore, in order to reduce power consumption of a light-emitting element, a method has been proposed, in which a metal oxide having a high work function is used for one of a pair of electrodes, through which light is not extracted, to reduce voltage loss due to the electrode and thus to reduce the drive voltage of a light-emitting element (e.g., see Patent Document 2).